(a) Field of the Invention
The present invention relates to a sensorless BrushLess Direct Current (BLDC) motor. More specifically, the present invention relates to a motor driving circuit and method for improving the efficiency of motor drive.
(b) Description of the Related Art
In driving a BLDC motor, current switching for each phase must be performed in a proper time in order to form a continuous rotor field of the BLDC motor, and a position of the rotor must be detected in order to properly perform current switching Therefore, devices for detecting the position of the rotor are needed, and among the devices in use are hall sensors or resolvers. However, the use of the sensors or resolvers involves a complicated circuit, and thereby, BLDC motors without hall sensors have been developed so as to decrease the inconvenience caused by appending hall sensors to motor control circuits.
BLDC motors without hall sensors detect the position of the rotor using the back electromotive force (BEMF) that results from the rotor rotation. However, when the motor starts to drive, the BEMF required for the check of rotor position needs to be generated. Therefore, a start up interval is necessary. The start up interval represents an initial and forced rotation period where the motor is rotated but the output is not engaged while the BEMF is generated.
In a sensorless BLDC motor, a start up interval is needed when the motor is still at standstill, but once the motor is rotating and generating BEMF on its own, the start up mode is no longer needed. However, just as in the case with conventional sensorless BLDC motors that restart after their power has been turned off, a disadvantage to efficiency can be found in that although the BEMF is generated the motor is rotating in the start up interval. That is to say, due to the need to rotate the motor in the start up forced drive period, both time and energy are wasted.